I was just about to create a forum thread about this, but everything I was going to suggest has already been said here.
- The ability for an artist to choose if they will allow users to add tags
- A slight separation or distinction between tags added by users and the artist, "suggested tags"
The issues of large galleries having the potential for tag abuse have already been brought up. So... yeah. Wow the community here is very thoughtful and helpful. I don't have anything new to say here.